1. Field of the Invention
This invention relates generally to a spin momentum transfer random access memory (SMT-MRAM) cell formed in either a magnetic tunneling junction (MTJ) or GMR Spin Valve (SV) configuration, using a spin momentum transfer (SMT) effect to change the magnetization of a free layer. In particular, it relates to a free layer design to enhance the properties of such a cell.
2. Description of the Related Art
The conventional magnetic tunneling junction (MTJ) or GMR Spin Valve (GMR/SV) device is a form of ultra-high magnetoresistive (MR) device in which the relative orientation of the magnetic moments of parallel, vertically separated magnetized layers, controls the flow of spin-polarized electrons tunneling either through a very thin dielectric layer (in the MTJ case) or through a thin conducting layer of a transitional metal (in the GMR/SV case) formed between those layers.
When injected electrons pass through the upper magnetized layer they are spin polarized by interaction with the magnetic moment of that layer. The majority of the electrons emerge polarized in the direction of the magnetic moment of the upper layer, the minority being polarized opposite to that direction. The probability of such a polarized electron then tunneling through the intervening tunneling barrier layer into the lower layer then depends on the availability of quantum states within the lower layer that the tunneling electron can occupy. This number, in turn, depends on the magnetization direction of the lower electrode. The tunneling probability is thereby spin dependent and the magnitude of the current (tunneling probability times number of electrons impinging on the barrier layer) depends upon the relative orientation of the magnetizations of magnetic layers above and below the barrier layer. The MTJ device can therefore be viewed as a kind of multi-state resistor, since different relative orientations (e.g. parallel and antiparallel) of the magnetic moments will change the magnitude of a current passing through the device.
In the GMR/Spin Valve form, the electrons are still spin-polarized by passage through a first layer having its magnetization set in a given direction, but now there is no tunneling barrier layer to provide a transport probability and corresponding current variation. Instead, the two spin orientations of the electrons produced in the first layer have differing scattering cross-sections in the second layer, depending upon the magnetization direction of that second layer. Thus, whether the device is MTJ or GMR the resulting effect is to produce a variable resistance depending upon the relative orientations of layer magnetization.
In a common type of device configuration one of the magnetic layers has its magnetic moment fixed in spatial direction (the pinned or reference layer) by exchange coupling to an antiferromagnetic layer, while the other magnetic layer has its magnetic moment free to move (the free layer) in response to some external magnetic fields. When utilized as a magnetic storage device, the magnetic moment of the free layer is made to switch its direction from being parallel to that of the pinned layer, whereupon the tunneling current is large, to being antiparallel to the pinned layer, whereupon the tunneling current is small. Thus, the device is effectively a two-state resistor. The switching of the free layer moment direction, called “writing,” is accomplished by external magnetic fields that are the result of currents passing through conducting lines adjacent to the cell.
In MRAM applications, the magnetizations of the stack of magnetic layers, whether it be an MTJ or GMR/SV form (to be called more simply the SV form hereinafter), can have two basic configurations, depending on whether the magnetization of the layers is in the plane of the layers (in-plane) or perpendicular to the plane of the layers. For the in-plane configuration, the magnetizations of both the free and pinned layers will remain in the layer plane as shown in FIG. 1a and FIG. 1b. 
Referring to FIG. 1a and FIG. 1b, there is shown a highly schematic drawing of a prior art exemplary MTJ cell (100). The cell is shown in an overhead (FIG. 1a) and side (FIG. 1b) cross-sectional view. In the overhead view, the cell is shown as having an elliptical horizontal cross-section, which is an advantageous shape for providing the magnetic layers with a shape induced magnetic anisotropy, which is a predisposition for the magnetization vector of the cell to lie along a particular direction. In this case, the predisposed direction is the major axis of the cell layer, which is then called the easy axis. Two arrows are drawn to represent two possible orientations of the magnetization of this particular cell layer along its easy axis, the assumption in this drawing being that the layer shown is the free layer, so the magnetization has the possibility of being in either direction along that axis. Note that the magnetization of the free layer could lie along other directions in the plane of the layer under certain circumstances, but the easy axis is the most stable and energetically preferable axis in the absence of external stimuli.
Referring now to FIG. 1b, there is shown a side cross-sectional view of the same cell as shown in FIG. 1a. The upper layer (50) is the free layer and the same two arrows are shown to indicate the two directions of magnetization along the easy axis. The lower layer (70) is the pinned layer and the magnetization is fixed (pinned) along its easy axis in a single direction. Typically, the pinning of such a layer is accomplished by interaction with adjacent layers that are not shown in this figure. The intermediate layer (60) is a tunneling barrier layer, which is a very thin dielectric layer through which electrons could pass by means of quantum mechanical tunneling.
Referring to FIG. 2 there is shown a side cross-sectional view of another cell configuration in which the magnetization directions are perpendicular to the planes of the cell layer. The free layer of this cell (50) has the two magnetizations shown as vertical arrows, while the pinned layer (70) has only one direction of its magnetization, indicated by a single upward arrow. In this perpendicular magnetization configuration, the predisposition to up and down orientations of the layer magnetization is not the result of shape anisotropy as in FIG. 1a, but would be provided by a crystalline anisotropy, the arrangement of the molecules in the layer resulting from applying a magnetic field during formation of the layer. The horizontal cross-section of this cell need not be elliptical and would probably be of some shape that does not compete with the vertical anisotropy.
The storage of digital information is provided by the orientation of the free layer magnetization, in-plane or perpendicular to the plane. Referring to FIG. 3, there is shown a schematic graph illustrating the resistance of such a cell (the cell of FIG. 1 or FIG. 2) as a result of an external field directed along the orientation of the pinned layer magnetization. Resistance is plotted vertically, field is plotted horizontally. When the field is off (zero field), the two states with minimum and maximum resistances correspond to the free layer magnetization being parallel to and anti-parallel to the pinned layer direction respectively. Hs is defined as the switching field, which is the strength of the external field required to change the direction of the free layer magnetization. The magnitude of this field is determined by the anisotropy energy of the cell element.
FIG. 4 is a highly schematic drawing showing an overhead view of an array of conventional MRAM cells formed between orthogonal word (200) and bit (300) lines. Each cell (100) is drawn with a slightly elliptical horizontal cross-section because such a shape anisotropy produces a magnetic anisotropy within the free layer that assists its magnetic moment in retaining a thermally stable fixed position after switching fields have been turned off. According to the diagram, the word line field will be along the short or “hard” axis of the cell, the bit line field will be along the easy axis (the longer axis of the ellipse) along which the magnetic moment of the cell spontaneously forms.
As shown in the figure, in conventional MRAM applications two orthogonal external fields are used to program an MRAM cell in the array. These fields are generated by an orthogonal matrix of current carrying lines, the word (200) and bit (300) lines. The bit lines provides the easy axis fields (along the major axis of the ellipse) and the word lines provide the hard axis fields (along the minor axis of the ellipse). To program a cell (101), currents are applied to both its word and bit lines and the combined fields overcome the shape anisotropy of the cell which tends to maintain the direction of the cell's magnetic moment in the absence of external fields. The cell whose magnetization direction is to be changed is called the selected cell (101) and the intersection of currents in the lines above this cell cause its magnetic moment to change. To program a cell (set its magnetic moment direction), both word and bit lines that intersect its position carry a current. However, there are many other cells (102), (103) that separately lie beneath the bit and word lines that intersect at the selected cell and these cells are not intended to be programmed. These cells experience the magnetic fields of either the word line or the bit line separately, which produces a lesser effect than the combined fields at the intersection of the lines. However, these cells can still be accidentally programmed, causing an error. Such cells are referred to as half-selected cells. Another shortcoming of this approach to programming cells is a scaling difficulty: as cell dimensions become smaller, Hs increases, requiring a higher current to produce the required switching.
The two shortcomings noted above, half-selected cells and increasing current requirements, can be avoided using the spin torque transfer (STT) switching mode. This mode is described in J. C. Slonczewski, “Current-driven excitation of magnetic multilayers,” J. Magn. Magn. Mater., vol. 159, pp. L1-L7, 1996 and M. Hosomi et al., “A Novel Nonvolatile Memory with Spin Torque Transfer Magnetization Switching: Spin-RAM” IDEM, 2005. It is also to be found in Sun, et al., U.S. Pat. No. 6,130,814.
Referring to FIG. 5, a portion of an MRAM device is schematically illustrated. The device incorporates two exemplary spin torque transfer (STT) MTJ cell elements, (500) and (600), formed between a common bit line (700) and circuitry (800), (900) for accessing each cell and causing a current to pass through it. In this general form, the elemental cells could be MTJ or GMR, depending on the nature of the intermediate layer (530), (630).
Each cell element comprises a capping layer (510), (610) that electrically contacts the common bit line (700), a free layer (520), (620) having a magnetization that can be varied, a tunneling barrier layer (530), (630), which could be a conducting intermediate layer in the case of a GMR cell, a pinned layer (540), (640) having a magnetization direction that is fixed, a seed layer (550), (650) and a bottom electrode (560), (660). Also shown is an electrical connection to the bottom electrode and a control circuit element (800), (900) that enables a current to flow from the bit line, through the cell and to ground. It is noted that a cell element operating on the SV (spin valve) GMR effect rather than MTJ effect could also be used in such a circuit, but the MTJ tunneling barrier layers (530), (630) would be replaced by electrically conducting layers. The operation of the MRAM device is based on the fact that the free layers (520), (620) of the individual cell elements have a magnetization that can be directed either along the magnetization direction of their pinned layers or opposite to that fixed magnetization. Moreover, the circuitry enables the relative direction to be sensed, typically by a resistance measurement of the cell, so the relative direction can be used to define a logical 1 or 0. As long as the relative directions remain stable and unchanged, the cell element is said to be storing information. When the relative direction is sensed by the circuitry, the stored information is being read. To write information of the cell element, the present state of its magnetization must be reversed, which is accomplished by passing a current, the critical switching current Ic, through the cell.
The critical switching current, Ic, is given by:Ic=CMs2V,  (1)Where C is a constant, Ms is the spontaneous magnetization of the free layer of either cell (510), (610), V=At is the volume of each cell, where A is its horizontal cross-sectional area and t is the free layer thickness.
From equation (1), it is obvious that the switching current, Ic, scales according to the cell dimensions. However, as the cell dimensions become smaller, thermal agitation may perturb the stored information (i.e., change the magnetization of the free layer). The effect of thermal agitation is given by the equation:f=f0exp{−BHsMsV/kT},  (2)where f is the thermal switching frequency, f0 and B are constants, k is Boltzmann's constant and T is the absolute temperature. For the stored information to be thermally stable (low switching frequency, f) the numerator of the exponentiated fraction, BHsMsV, must exceed a certain constant value. As the dimensions of the cell scale down, the area factor, A, in the volume V=At decreases, so to maintain the value of the numerator, Hs and/or Mst must increase. But increasing Mst will increase Ic, which is undesireable. In addition, for a tunneling MTJ, the voltage across the dielectric layer is: Vc=Ic R, so it is important to have a low value of the critical switching current, Ic, to avoid the reliability concerns of dielectric breakdown. Thus it is a challenge to have a low value of Ic for reliable writing, yet to maintain the thermal stability of stored data. For such reasons, a new STT-MRAM device configuration is proposed herein.